I don't think what Duckear posted reflects accurate medical biochemistry. At 300 cal/day you will be in a state of progressive malnutrition. The fat stores may spare (not stop) protein catabolism early on, but with time (weeks) the person will develop signs of marasmus after about 1/3 of protein stores are depleted. The attached handout is what physicians in training will learn.

http://www.uic.edu/depts/mcam/mcbc/lect_2004/lecture_41_43.pdf